1. Field of the Invention
This invention relates to liquid crystal (LC) displays which operate with a dominant non-polarization rotation effect (PRE), and more particularly to LC displays based primarily upon birefringence or dye-doped absorption, together with operation below the LC's lowest PRE transmission maximum.
2. Description of the Related Art
Conventional 90.degree. twisted nematic LC displays employ a polarizer in front of the LC cell and an analyzer behind the cell that is rotated 90.degree. with respect to the polarizer. In the absence of an applied electric field across the cell, the polarization of input light is twisted 90.degree. as it progresses through the cell, allowing for maximum optical transmission. (A 270.degree. twist achieved with the addition of a chiral dopant to the LC is a conventional arrangement that can also be used. For purposes of this application, since a 270.degree. twist results in essentially the same twisted polarization plane as a 90.degree. twist, a 270.degree. twist will be considered to be equivalent to and included within the general term 90.degree. twist.) When full voltage is applied across the cell the LC directors, which originally were parallel to the cell boundaries, rotate to align with the field at right angles to the cell boundaries. This removes the twisting effect of the LC, leaving the polarization direction of input light unchanged as it traverses the cell and thus causing the analyzer to block the light transmission. This type of system is described, for example, in Schadt and Helfrich, "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal", Applied Physics Letters, Vol. 18, No. 4, Feb. 15, 1971, pages 127-128. It is the commonly used system for transmission mode LC displays, such as notebook computers. However, it is not well suited for reflective mode operation because of its poor brightness.
To obtain a high contrast ratio it is desirable to maximize the cell's transmission when zero voltage is applied. The cell's transmission T with the LC directors perpendicular to the electric field is given by the equation: ##EQU1## where u=2d.DELTA.n/.lambda., d is the cell thickness, .DELTA.n is the difference between the LC's extraordinary and ordinary indices of refraction and .lambda. is the input light wavelength. This equation is derived from Gooch and Tarry, "The optical properties of twisted nematic liquid crystal structures with twist angles .ltoreq.90.degree.", J. Phys. D:APPL. Phys., Vol. 8, 1975, pages 1575-1584.
The lowest value of u to satisfy the requirement T=1 is .sqroot.3, at which d.DELTA.n is approximately equal to 0.866.lambda.. Assuming .lambda.=0.55 microns (micrometers) and using a typical value of 0.1 for An yields a cell thickness d of 4.8 microns. Twisted nematic cells with a thicker LC layer would result in slower response times.
Twisted nematic cells of this type are commonly used for transmissive mode displays. However, the transmissive mode has a number of drawbacks. It requires a backlight for illumination which typically consumes the major portion of the available battery power. Also, transmissive mode displays are not suitable for reflection-type applications, such as direct view displays that utilize ambient light or projection displays. Reflective mode displays generally reduce power consumption, while at the same time they tend to boost brightness.
When a conventional twisted nematic LC cell of the type described above is used for reflective mode operation, however, the light modulation efficiency is greatly sacrificed. In a conventional projection scheme the input light is polarized and redirected 90.degree. by a polarizing beam splitter (PBS), from which the light traverses the LC cell and is reflected back through the cell towards the PBS by a mirror on the other side of the cell. Since the polarized light is restored to its original polarization direction by traversing the LC cell twice in opposite directions, it is diverted by the PBS back towards its source, rather than transmitted as a projection output. This results in a relatively low net transmission for cell voltages below the LC tilt threshold.
As the applied cell voltage exceeds the threshold level, the PRE is broken and a small amount of light modulation occurs during the directors' reorientation. At voltages significantly greater than the threshold level, the LC directors are aligned parallel to the electric field so that the light polarization is not rotated and the PBS transmission therefore approaches zero. Thus, a twisted nematic cell operated to maximize T in equation (1) is not suitable for reflective mode operation.
One application of the Gooch and Tarry approach of selecting the LC cell parameters to maximize T for low modulation voltages is described in U.S. Pat. No. 4,398,803 to Pohl et al. This patent seeks to improve the viewing angle by setting d.DELTA.n within the range of 0.15-0.6 microns. Like the other references described above, this patent is intended for transmissive rather than reflective mode applications, and relies upon the PRE. While the viewing angle is apparently improved, the brightness is significantly reduced as the cell thickness approaches the lower end of the stated range. Thus, "optimum properties" were noted for cells with d.DELTA.n within the range of 0.45-0.50 microns.
Liquid crystal modulators that operate through the birefringence effect rather than PRE are also known. As opposed to PRE systems, in which the input light is polarized parallel to the input LC directors, the birefringence effect relies upon a substantial angle between the polarizations of the input light and the input LC directors, with a maximum modulation achieved at 45.degree.. Utilization of the birefringence effect relies upon a difference between the ordinary and extraordinary phase retardations, rather than an LC twist. Ignoring constants, an equation for T in a birefringent operation can be obtained from Born and Wolf, Principals of Optics--Electromacnetic Theory of Propagation, Interference and Diffraction of Light, 6th ed., Pergamon Press, 1980, pages 694-696: EQU T=Sin.sup.2 2.beta.Sin.sup.2 .delta.2 (2)
where .beta. is the angle between the input light polarization and the input LC directors, and .delta.=2.pi.d.DELTA.n/.lambda.. It can thus be seen that the birefringence effect is strongly dependent upon the wavelength .lambda.; optimizing the system for one wavelength requires a significant sacrifice of efficiency for other wavelengths. The contrast ratio is decreased dramatically as the bandwidth of light increases.
A third type of LC light modulator involves doping the LC with dichroic dyes. This establishes a "guest-host" relationship in which the dye molecules align with the LC directors to absorb polarizations parallel to the cell's input face in the quiescent state. When a voltage is applied to the cell sufficient to rotate the LC directors to a right angle to the input cell face, the dye molecules rotate along with the LCs and allow a transmission through the cell. The need for an external polarizer is thus eliminated. One such system is described in Cole and Kashnow, "A new reflective dichroic liquid-crystal display device", Applied Physics Letters, Vol. 30, No. 12, Jun. 15, 1977, pages 619-621. However, it is suited only to a reflective mode operation, and requires the addition of a quarter-wave plate.